The invention relations to a method for setting register on a multicolor printing machine having color printing units assigned to various printing inks, at least one image cylinder, image production equipment for producing color separations, in particular electrostatic latent color separations, on the at least one image cylinder, a carrier for printing substrates and at least one image transfer point for the transfer of the color separations to the printing substrates, an assignment of the image production points on the at least one image cylinder being carried out in order to achieve coincidence of register between the color separations in the print, by both the production of the image starts and the production of areas of the color separations being set on the basis of the print and the evaluation of register marks.
The invention also relates to apparatus for setting register in accordance with the above-mentioned method on a multicolor printing machine having color printing units assigned to various printing inks, at least one image cylinder, image production equipment for producing color separations, in particular electrostatic latent color separations, on the at least one image cylinder, a carrier for printing substrates and at least one image transfer point for the transfer of the color separations to printing substrates, sensors for measuring positions and at least one setting device for assigning the positions of the image production points on the at least one image cylinder in order to achieve coincidence of register between the color separations in the print, at least one register sensor being arranged to detect register marks, and the setting device being designed such that it initiates the printing of register marks and evaluates them in such a way that it sets the production of the image starts and the production of areas of the color separations on this basis.
The invention also relates to a multicolor printing machine having apparatus of the aforementioned type.
Printing color illustrations, in particular color images, is carried out by a number of color separations being printed over one another. These are generally the colors yellow, magenta and cyan, as well as black. If required, special colors are also added. By means of overprinting these colors, all color compositions can be achieved, the quality of the prints depending significantly on the in-register overprinting of the color separations. In the case of conventional printing processes, the printing plates are corrected by means of test prints and register marks printed together with the latter until exact overprinting, that is to say the maintenance of register in the print is achieved.
In digital printing processes, the image cylinders are written with image points by means of an item of image production equipment in each case, for example, by electrostatic charges being generated and these being provided with adhering color pigments. The color pigments are then transferred to a printing substrate. In the case of digital printing processes, maintenance of register can be achieved by the image production equipment being controlled appropriately. Since the setting of an image is carried out anew for each print, it is not necessary, as in conventional printing processes, for a one-off setting to be made. Instead, provision can be made for presetting and regulation, which makes corrections for each individual print. Of course, this applies not only to the application of electrostatic latent images but also to all other printing processes in which image points are applied by means of a digital control system.
For an electrostatic printing process of the type mentioned at the beginning, U.S. Pat. No. 5,287,162 has, therefore, proposed printing register marks, preferably onto the carrier for the printing substrates and detecting these by means of an apparatus. In this case, the times which the register marks need from the production by the image production equipment as far as a detection point are determined. These times are then used to determine the instants at which the image production equipment begins its image-setting on the individual image cylinders in order to achieve the maintenance of register after the images have been transferred to a printing substrate. Faults, which can be produced in a manner, which repeats over time, by fluctuations in the speed of the image cylinders. Faulty determinations of the position by the rotary encoders of the image cylinders or other causes are registered by means of calibration tables. On the basis of these calibration tables, compensation for the faults is carried out. Provision is made for either the regulation of the image cylinder speeds, as a function of the position of the image cylinders, or corresponding production of the lines in the image.
Although, in the above-mentioned specification, mention is made of the fact that the start of the data, that is to say, the beginning of the image setting, and then the start of each line in the image, are controlled, what is lacking is the solution to the problem of obtaining data for the image starts and lines in the image or other subareas of a color separation in such a way that a rapid correction with high quality can be achieved with a tolerable outlay.
The invention is, therefore, based on the object of developing methods and apparatus of the type mentioned at the beginning in such a way that the setting of register can be implemented with a tolerable computing effort and high accuracy and, in particular, the data can be obtained and processed with a tolerable computing effort.
With regard to the method, the object is achieved by at least one picture page being printed which has register marks and has a length differing from that of the prints whose register is to be set, and in order to determine the correction values, by the data from the register marks of the at least one picture page being measured and placed in a relationship with positions of elements that carry images and substrates, and by the image production equipment being controlled in accordance with the correction values.
With regard to the apparatus, the object is achieved by its having at least one setting device which is designed such that it initiates the printing of at least one picture page which has register marks and has a length differing from that of the prints whose register is to be set, and by the at least one setting device being designed such that, in order to determine the correction values, it places the data from the detected register marks in a relationship with positions of elements that carry images and substrates, and controls the image production equipment in accordance with these correction values.
With regard to the multicolor printing machine, the object is achieved by its being equipped with apparatus of the above-mentioned type.
The invention has the advantage that it provides a method and apparatus by means of which, with a tolerable computing effort and within a short time, the correction values can be determined from the actual and desired values, and the corrections can be made. The basic idea of the invention consists in the ability to separate the actual values of the image starts from the actual values of the areas of the color separations by means of the proposed configuration of register marks. This separation can be carried out as an intermediate step, for example, for the determination by computation of the correction values for the image productions, or separate correction values can be determined and taken into account.
To achieve the aforementioned ability to be separated, it is proposed that at least one test print be made in which a picture page is printed which has register marks and which is longer than the prints whose register is to be set, or, if a number of picture pages are set up, that these be shorter than the prints whose register is to be set. Length deviations of this type make it possible to separate the faults in the image starts, which are caused by an assignment error between the image productions, from the faults in the image areas, which are caused by transfer faults in the color separations produced from the image production, as far as their transfer to a printing substrate. These different causes of the faults permit their separation, since the linking of the faults in the image starts with the faults in the areas of the color separations may be eliminated by means of the picture pages which have register marks and are of a length differing from the prints to be set up. This separation is necessary, at least within the calculation of correction values, in order to implement a register setting with a tolerable computing effort and high accuracy. The separation, to be performed at least as an intermediate computing step, is possible on the basis of the different characteristics of the faults, since the fault in the areas of the color separations occurs as an essentially periodic curve, and the fault in the image starts can be detected as an intercept not equal to zero (offset) of this curve.
However, the difficulty of this detection is that the average value of the curve in actual fact does not have a straight course either. In this way, the measured curve can be the superimposition of two curves, which have to be separated from each other in order to detect the faults in the image starts and the faults in the areas of the color separations. This can be carried out, for example, by means of a computer, which recognizes the characteristics on the basis of an algorithm. This separation is possible as a result of the different lengths of the picture page or picture pages relating to the prints to be set up, since as a result the calculation is possible and can be carried out more simply with fewer unknowns.
A further problem is that, on the one hand, the shape of the curve must be capable of being reconstructed on the basis of point-by-point measurements in such a way that the above-described analysis of the curve is possible, but, on the other hand, no drift must distort the analysis of the curve. The printing of picture pages with register marks must, therefore, satisfy the condition that, by means of the register marks, sufficient measured points are available for the reconstruction of shape and position. In contrast, drift, for example, as a result of a change in the machine temperature, can be eliminated by computation. This is possible, or at least facilitated, as a result of the invention, since, because of the different lengths of the picture pages, the computation can be carried out with fewer unknowns. Once the data from the curve have been registered, an assignment of the image productions to positions on the image cylinders can be carried out in such a way that both the image starts and the areas of the color separations arrive in their desired position on the carrier or on a printing substrate located on the latter. At the same time, the assignment of the data from the register marks to positions of the elements that carry images and substrates can be carried out as a direct position assignment or as an indirect position assignment via recorded times.
A development of the method provides for the data from the register marks to be placed in a relationship with positions of the image cylinders and of the carrier, and for the image production equipment on the image cylinders, to be controlled in order to achieve the desired values on the carrier. With regard to the apparatus, it is proposed that the at least one setting device be designed such that it places the data from register marks in a relationship with positions of the image cylinders and of the carrier and uses these to control the image production equipment on the image cylinders in order to achieve the desired values on the carrier. This configuration is primarily for multicolor printing machines, in which the color separations are transferred directly from the image cylinders to printing substrates, which are located on a carrier.
If a printing machine is configured in such a way that image transfer cylinders are arranged between the image cylinders and the carrier for printing substrates, then, in addition to the measure just mentioned, it is proposed that the data from the register marks also be placed in a relationship with positions of the image transfer cylinders and included in the control of the image production equipment. With regard to the apparatus, it is proposed that the at least one setting device be designed such that it places the data from the register marks in a relationship with positions of the image transfer cylinders, as well, and includes it in the control of the image production equipment. This measure is particularly important, since, for the necessary contact between image cylinder and image transfer cylinder, at least one of the surfaces may have a certain elasticity. Provision is generally made for the image transfer cylinder to be equipped with an elastic cover. As a result of the deformation of an elastic surface of this case in the type of two cylinders at whose point of contact the transmission of force takes place, overdrive of one cylinder with respect to the other occurs, since the behavior of the elastic material influences the transmission ratio. In order to eliminate this fact, as well as any nonroundness or other irregularities of the image transfer cylinder, it is necessary for the data from the register marks also to be placed in a relationship with the image transfer cylinders and taken into account in the control of the image production equipment as correction values assigned to the positions of the image transfer cylinders.
The method is expediently configured in such a way that separate correction values are determined for the production of the image starts and the areas of the color separations, the production of the areas depending on the image starts. With regard to the apparatus, the at least one setting device is designed in a corresponding way. If, given a corresponding control system, an image start is positioned exactly, then the areas of the color separations can follow this, it being possible for each individual area to be positioned in accordance with the correction values for the areas of the color separations.
The at least one picture page with register marks can have an extra length, it then being possible under certain circumstances for a single picture page to suffice for determining the correction values. Another possibility is for a number of picture pages that are shorter than the prints whose register is to be set, be set up. In the first-mentioned case, it is expedient if the length of a picture page is selected such that a number of periods of faults of areas of color separations are detected. In the second case, it is expedient if the shorter picture pages are positioned such that they contain a number of periods of faults of image starts. Since the faults relating to the image starts and the faults relating to the areas of color separations generally exhibit a periodic variation, these periods can be measured, and, in this way, it is easily possible to classify the data from the register marks, that is to say, the measured actual values, in accordance with actual values relating to the image starts and in accordance with actual values relating to the areas of color separations, and to carry out the control in the above-mentioned way. With regard to the apparatus, provision is then made for the at least one setting device to be designed for the evaluation in the aforementioned way.
For the method, it is proposed that the register marks be designed in such a way that the actual positions of the image starts can be detected from them and that the correction values be determined from a comparison between the actual and the desired positions. In addition, it is proposed that the register marks be designed in such a way that the actual positions of the areas can be detected from them and that the correction values be determined from a comparison between the actual and the desired positions. With regard to the apparatus, it is proposed that it be designed such that it determines the correction values in the aforementioned way from the data from the detected register marks.
The areas of the color separations can be lines of image points or groups of lines of image points. The apparatus is then correspondingly designed for the calculation of areas, which can be lines of image points or groups of lines of image points.
In order to assign the areas to positions of elements that carry images and substrates, there are various possibilities. Firstly, the position assignment can be carried out indirectly via a time assignment, or it can be performed in a direct way as a position assignment. In the latter case, as a particularly expedient configuration, it is proposed that the position of the areas be assigned to defined angular positions of the image cylinders. With regard to the apparatus, it is proposed that it be equipped with at least one sensor for at least one position measurement and be designed such that it assigns the position of the areas to defined angular positions of the image cylinders. In this case, it is particularly expedient to define angular sequences of the image cylinders permanently and to assign the sizes of the areas to these defined angular sequences. The apparatus is also designed for a corresponding assignment. This assignment to defined angular sequences of the image cylinders is a simple method for eliminating different transfers of the areas in different positions of the elements transferring them during the production of the color separations in such a way that the accuracy of the register on the printing substrate is subsequently ensured. Simple correction of the size of the areas can then be made by changing the spacing of the lines of image points.
Further possibilities for determining the correction values for the image starts or for the areas of the color separations consist in the position of the image starts being assigned to the position of specific areas spaced apart in a defined way or, in the other case, in the correction values for the areas being determined by the actual values of the position of many image starts. The apparatus is then designed for an appropriate assignment or determination.
The method is preferably configured in such a way that, from the data from the register marks, the separate acquisition of the actual values of the image starts and of the actual values of the areas of the color separations is carried out by the data obtained by detecting the register marks being separated by means of appropriate algorithms on the basis of their different characteristics. For this purpose, it is proposed that the apparatus contain a computer which is loaded with algorithms, by means of which it separates the data (16) obtained by means of the detection of the register marks (9, 9xe2x80x2, 9xe2x80x3, 9xe2x80x2xe2x80x3) on the basis of the different characteristics of the actual values of the image starts (10) and the actual values of the areas (10xe2x80x2, 10xe2x80x2xe2x80x3, . . . 10n). Precisely for this configuration, the measure according to the invention, with the picture pages with lengths differing from those of the prints, offers the advantage that the calculation can be carried out with a tolerable effort and quickly enough. Situations in which a separation of this type is not possible are ruled out by the measures according to the invention.
Provision may be made for the test print of the at least one picture page to be made before a printing substrate is printed, in order to preset the register, and the calculation is used to check that the first substrate printed is already completely inregister. In this case, the calculation of the correction values according to the invention offers the major advantage that machines can be designed in such a way that different individual pages can also be printed completely inregister. Here, it is expedient if the test print is printed onto the carrier for printing substrates and is removed again after the evaluation. For this purpose, the apparatus is equipped with a device for removing test prints of picture pages from the carrier.
Of course, however, for register control, a test print can also be made during the printing of the printing substrates, it being possible for the test prints to be applied for example to areas of the carrier that are not covered by printing substrate and to be removed again after the evaluation. This measure has, for example, the advantage that a drift correction can be recorded during the print and counteractive control by means of appropriate correction values can be performed immediately.